1. Field of Invention
The invention relates to a process for forming a powder coating composition, and more specifically to an emulsion aggregation and coalescence process for the preparation of powder coating compositions.
2. Description of Related Art
Powder coatings form a rapidly growing sector of the coatings market. Powder coatings are utilized for either decorative purposes or protective purposes or both. For decorative purposes, color, gloss, and appearance may be the primary attributes. Most decorative coatings are thin coatings mainly using thermoset coatings. For protective purposes, the coatings should have longevity, corrosion protection, impact resistance properties and serve as insulation. In this latter role, the coatings should be thick and can be applied in powder form.
Powder coating materials are solid compositions which are generally applied by an electrostatic spray process in which the powder coating particles are electrostatically charged by the spray gun and the substrate (normally metallic) is earthed. The charge on the powder coating particles is normally applied by interaction of the particles with ionized air (corona charging) or by friction (tribostatic or “tribo” charging). The charged particles are transported in air towards the substrate and their final deposition is influenced inter alia by the electric field lines that are generated between the spray gun and the work piece, as well as the space charge electric field from the charged powder cloud.
Powder coating compositions also may be applied by fluidized-bed processes, in which the substrate work piece is preheated (typically to 200° C.-400° C.) and dipped into a fluidized bed of the powder coating composition. The powder particles that come into contact with the preheated surface melt and adhere to the work piece. In the case of thermosetting powder coating compositions, the initially-coated work piece may be subjected to further heating to complete the curing of the applied coating. Such post-heating may not be necessary in the case of thermoplastic powder coating compositions.
Another alternative application technique for powder coating compositions is the so-called electrostatic fluidized-bed process, in which the fluidizing air is ionized by means of charging electrodes arranged in the fluidizing chamber or, more usually, in the plenum chamber below the porous air-distribution membrane. The ionized air charges the powder particles, which acquire an overall upwards motion as a result of electrostatic repulsion of identically charged particles. The effect is that a cloud of charged powder particles is formed above the surface of the fluidized bed. The substrate work piece (earthed) is introduced into the cloud and powder particles are deposited on the substrate surface by electrostatic attraction. No preheating of the substrate work piece is required.
The processes described above generally employ powder compositions obtained by extrusion and pulverization methods. Powder compositions thus produced have numerous drawbacks. The powder compositions consist of irregularly shaped particles in a wide range of sizes.
Powder coating compositions formed by these conventional methods produce coatings that may be unnecessarily thick, due to the size of the particles of the powder coating compositions. In addition, the thickness of these coatings may be non-uniform due to the wide range of particle sizes and the irregular shapes of the particles of the powder coating compositions.
Thus, there exists a need for processes to produce powder coating compositions comprising very small, regularly shaped particles having a narrow range of sizes. There further exists a need for processes for providing a thin film coating of uniform thickness to a substrate by means of a powder coating composition. There also exists a need for a process for forming powder coating composition particles of a small size and narrow size range, with improved economic feasibility. Thus there further exists a need for powder compositions formed chemically in situ and which do not require known pulverization and/or classification methods.